This invention relates to an apparatus for detecting an alignment pattern for a mask aligner or the like used in the production of large scale integration (LSI) devices.
Although this invention is applicable to mask aligners in general as will be described later, it is especially effectively used with a reduction-projection mask aligner (hereinafter referred to as RPA) as disclosed in U.S. patent application Ser. No. 771,201. The RPA will be first described below.
As an alignment pattern used with this RPA, at least three linear segments 2 perpendicular to each other are formed, for example, in the peripheral portion of a wafer 1 radially of the center thereof, as shown in FIG. 1A. In this pattern, when a photoresist 26 is coated by rotating the wafer 1, the photoresist only slightly moves outward by the centrifugal force but it is not displaced in a direction perpendicular to the longitudinal direction of the patterns as shown in FIG. 1B, resulting in a high accuracy of position detection. As seen from the foregoing description, the mask and the wafer are aligned to each other with the photoresist 26 0.8 to 1 mm thick coated on the wafer 1. As a result, when this coating is irradiated with a single-wavelength light ray such as the g-line having a wavelength of 0.43 .mu.m, a multiplicity of interference fringes are produced due to the varying thickness of the photoresist 26 covering the pattern 2 as shown in FIG. 2A. The interference fringes are scanned in the direction of arrow by a photo-electric converter element such as for detecting the brightness of the interference fringes through photo-electric conversion by scanning pin holes or slits, or a TV camera, and are converted into an electrical signal, thereby producing a video signal 4 as shown in FIG. 2B.
In conventional apparatuses as disclosed in U.S. Pat. Nos. 3,617,751 or 3,955,072, an appropriate threshold value 5 is set beforehand, on the basis of which the video signal is converted into a binary signal 6 as shown in FIG. 2C, so that the center 7 between the binary signal pulses 6, namely, the center of the segment of the pattern is determined.
By this method, however, it is impossible to process a video signal 4a associated with a small linear pattern segment of the pattern 2 which fails to reach the threshold value 5 as shown in FIG. 2D. Further, in the case of a linear segment of the pattern associated with a video signal 4b not completely symmetric as shown in FIG. 2E, the conversion of the video signal into a binary signal with reference to the threshold value 5 results in the production of a binary signal 6b as shown in FIG. 2F. The center 7b of the binary signal pulses 6b is greatly displaced from the true center of the linear pattern segments, thereby making it impossible to determine the right centers of the linear pattern segments.
Still another disadvantage of the conventional apparatuses is that in the case where noise 8 included in the video signal 4c reaches the threshold level 5 as shown in FIG. 3A, the binary signal 6c shown in FIG. 3B is obtained, which binary signal 6c has a center 7c slightly displaced from the true center of the linear pattern segments. Furthermore, when the video signal 4d is slightly asymmetric due to some defect of the linear patterns segments themselves as shown in FIG. 4A, conversion of the video signal 4d into a binary signal with reference to the threshold value 5 results in a binary signal 6d as shown in FIG. 4B, thus making impossible the detection of the right center of the linear patterns.